Exhaust gas recirculation (EGR) systems are employed in automotive vehicles to recirculate a controlled portion of the engine exhaust gas into an engine intake manifold to reduce emissions and improve fuel efficiency. Such systems typically employ an EGR valve that is disposed between the engine exhaust manifold and the engine intake manifold, and operable, when in an open position, to recirculate a portion of the exhaust gases from the exhaust side of the engine back to the intake side of the engine. In one arrangement, the EGR flow rate to the intake manifold is varied according to one or more conditions, such as engine temperature, air charge entering the intake manifold, and engine speed.
It is desirable to monitor operation of an EGR system through onboard diagnostic routines to determine whether or not the system is operating as expected. One approach to EGR monitoring in vehicles includes the use of a non-intrusive monitor. The non-intrusive EGR monitor requires operation at low load and high load with low amounts of EGR. These operating points are inefficient and as a result, hybrid engine operations typically avoid them, thus preventing the non-intrusive monitor from completing a diagnostic test. In contrast, an intrusive monitor only needs the highly efficient medium load points to complete a diagnostic test. However, in hybrid vehicles, the test results can be corrupted by intake variable camshaft timing (VCT) operation.
An EGR onboard diagnostic routine can be confounded by rapid VCT adjustments. A rapid change in VCT causes a manifold filling delay such that the mass air flow (MAF) into the intake manifold and the manifold absolute pressure (MAP) do not track each other. In non-hybrid vehicles, VCT is not employed aggressively, meaning that highly retarded valve timings are not used often and the rate of change in valve timing is typically modest. Thus, the problem of delay was not found to substantially impair the accuracy of the EGR onboard diagnostic routine in non-hybrid vehicles. Whereas, in hybrid vehicles, more aggressive use of VCT can be accommodated, both in highly retarded timings and rapid rates of change in valve timing. As such, there is a need to provide a robust and systematic means to monitor EGR systems in hybrid vehicles.